Subfunctionalization of cellulose synthases in seed coat epidermal cells mediates secondary radial wall synthesis and mucilage attachment

Genome-Wide Association Study Reveals the Genetic Basis of Crude Fiber Components in Brassica napus L. Shoots at Stem Elongation Stage

Brassica napus is currently the principal field crop for producing materials for primary, secondary and tertiary industries. B. napus shoots at stem elongation stage are rich in anthocyanins, vitamin C and mineral elements such as selenium, calcium and zinc, and represent a new type of green vegetable. However, the high crude fiber (CF) content of B. napus shoots affects their taste, and few studies have focused on the quality traits of these vegetables. In this study, we investigated five traits related to the CF components, including neutral detergent fiber (NDF), acid detergent fiber (ADF), acid detergent lignin (ADL), hemicellulose (Hem) and cellulose (Cel), of B. napus shoots. Whole-genome resequencing at a depth of ∼20× was utilized to genotype an association panel of 202 diverse accessions, which resulted in the identification of 6,093,649 single nucleotide polymorphisms (SNPs) and 996,252 indels, respectively. A genome-wide association study (GWAS) was performed for the five CF-related traits based on the phenotypic data observed in four environments. A total of 1,285 significant SNPs were detected at the threshold of -log10 (p) = 5.16, and 97 significant association regions were obtained. In addition, seven candidate genes located on chromosomes A2 (one gene), A8 (three genes), A9 (two genes) and C9 (one gene) related to CF traits were identified, and ten lines containing low CF contents were selected as excellent germplasm resources for breeding. Our results contributed new insights into the genetic basis of CF traits and suggested germplasm resources for the quality improvement of B. napus shoots.

When Size Matters: New Insights on How Seed Size Can Contribute to the Early Stages of Plant Development

The seed habit is the most complex and successful method of sexual reproduction in vascular plants. It represents a remarkable moment in the evolution of plants that afterward spread on land. In particular, seed size had a pivotal role in evolutionary success and agronomic traits, especially in the field of crop domestication. Given that crop seeds constitute one of the primary products for consumption, it follows that seed size represents a fundamental determinant of crop yield. This adaptative feature is strictly controlled by genetic traits from both maternal and zygotic tissues, although seed development and growth are also affected by environmental cues. Despite being a highly exploited topic for both basic and applied research, there are still many issues to be elucidated for developmental biology as well as for agronomic science. This review addresses a number of open questions related to cues that influence seed growth and size and how they influence seed germination. Moreover, new insights on the genetic-molecular control of this adaptive trait are presented.

Berberine bridge enzyme-like oxidases of cellodextrins and mixed-linked β-glucans control seed coat formation

Plants have evolved various resistance mechanisms to cope with biotic stresses that threaten their survival. The BBE23 member (At5g44360/BBE23) of the Arabidopsis berberine bridge enzyme-like (BBE-l) protein family (Arabidopsis thaliana) has been characterized in this paper in parallel with the closely related and previously described CELLOX (At4g20860/BBE22). In addition to cellodextrins, both enzymes, renamed here as CELLODEXTRIN OXIDASE 2 and 1 (CELLOX2 and CELLOX1), respectively, oxidize the mixed-linked β-1→3/β-1→4-glucans (MLGs), recently described as capable of activating plant immunity, reinforcing the view that the BBE-l family includes members that are devoted to the control of the homeostasis of potential cell wall-derived damage-associated molecular patterns (DAMPs). The 2 putatively paralogous genes display different expression profiles. Unlike CELLOX1, CELLOX2 is not expressed in seedlings or adult plants and is not involved in immunity against Botrytis cinerea. Both are instead expressed in a concerted manner in the seed coat during development. Whereas CELLOX2 is expressed mainly during the heart stage, CELLOX1 is expressed at the immediately later stage, when the expression of CELLOX2 decreases. Analysis of seeds of cellox1 and cellox2 knockout mutants shows alterations in the coat structure: the columella area is smaller in cellox1, radial cell walls are thicker in both cellox1 and cellox2, and the mucilage halo is reduced in cellox2. However, the coat monosaccharide composition is not significantly altered, suggesting an alteration of the organization of the cell wall, thus reinforcing the notion that the architecture of the cell wall in specific organs is determined not only by the dynamics of the synthesis/degradation of the main polysaccharides but also by its enzymatic oxidation.

Genetic dissection of monosaccharides contents in rice whole grain using genome-wide association study

The simplest form of carbohydrates are monosaccharides which are the building blocks for the synthesis of polymers or complex carbohydrates. Monosaccharide contents of 197 rice accessions were quantified by HPAEC-PAD in rice (Oryza sativa L.) whole grain (RWG). A genome-wide association study (GWAS) was carried out using 33,812 single nucleotide polymorphisms (SNPs) to identify corresponding genomic regions influencing neutral monosaccharides contents. In total, 49 GWAS signals contained in 17 genomic regions (quantitative trait loci [QTLs]) on seven chromosomes of rice were determined to be associated with monosaccharides contents of whole grain. The QTLs were found for fucose (1), mannose (1), xylose (2), arabinose (2), galactose (4), and rhamnose (7) contents, all of which are novel. Based on co-location of annotated rice genes in the vicinity of GWAS signals, the constituents of the whole grain were associated with the following candidate genes: arabinose content with α-N-arabinofuranosidase, pectinesterase inhibitor, and glucosamine-fructose-6-phosphate aminotransferase 1; xylose content with ZOS1-10 (a C2H2 zinc finger transcription factor [TF]); mannose content with aldose 1-epimerase-like protein and a MYB family TF; galactose content with a GT8 family member (galacturonosyltransferase-like 3), a GRAS family TF, and a GH16 family member (xyloglucan endotransglucosylase/hydrolase xyloglucan 23); fucose content with gibberellin 20 oxidase and a lysine-rich arabinogalactan protein 19, and finally rhamnose content with myo-inositol-1-phosphate synthase, UDP-arabinopyranose mutase, and COBRA-like protein precursor. The results of this study should improve our understanding of the genetic basis of the factors that might be involved in the biosynthesis, regulation, and turnover of monosaccharides in RWG, aiming to enhance the nutritional value of rice grain and impact the related industries.

Multi-omic analyses reveal the unique properties of chia (Salvia hispanica) seed metabolism

Chia (Salvia hispanica) is an emerging crop considered a functional food containing important substances with multiple potential applications. However, the molecular basis of some relevant chia traits, such as seed mucilage and polyphenol content, remains to be discovered. This study generates an improved chromosome-level reference of the chia genome, resolving some highly repetitive regions, describing methylation patterns, and refining genome annotation. Transcriptomic analysis shows that seeds exhibit a unique expression pattern compared to other organs and tissues. Thus, a metabolic and proteomic approach is implemented to study seed composition and seed-produced mucilage. The chia genome exhibits a significant expansion in mucilage synthesis genes (compared to Arabidopsis), and gene network analysis reveals potential regulators controlling seed mucilage production. Rosmarinic acid, a compound with enormous therapeutic potential, was classified as the most abundant polyphenol in seeds, and candidate genes for its complex pathway are described. Overall, this study provides important insights into the molecular basis for the unique characteristics of chia seeds.

Regulation of seed coat mucilage production and modification in Arabidopsis

The Arabidopsis seed coat mucilage is a polysaccharide-rich matrix synthesized by the seed coat epidermal cells. It is a specialized cell wall mainly composed of three types of polysaccharides (i. e. pectin, hemicellulose, and cellulose), and represents as an ideal model system for plant cell wall research. A large number of genes responsible for the synthesis and modification of cell wall polysaccharides have been identified using this model system. Moreover, a subset of regulators controlling mucilage production and modification have been characterized, and the underlying transcriptional regulatory mechanisms have been elucidated. This substantially contributes to the understanding of the molecular mechanisms underlying mucilage synthesis and modification. In this review, we concisely summarize the various genes and regulators involved in seed coat cell differentiation, mucilage biosynthesis and modification, and secondary cell wall formation. In particular, we put emphasis on the latest knowledge gained regarding the transcriptional regulation of mucilage production, which is composed of a hierarchal cascade with three-layer transcriptional regulators. Collectively, we propose an updated schematic framework of the genetic regulatory network controlling mucilage production and modification in the Arabidopsis mucilage secretory cells.

Mutation in the Endo-β-1,4-glucanase (KORRIGAN) Is Responsible for Thick Leaf Phenotype in Sorghum

Sorghum [Sorghum bicolor (L.) Moench] is an important crop for food, feed, and fuel production. Particularly, sorghum is targeted for cellulosic ethanol production. Extraction of cellulose from cell walls is a key process in cellulosic ethanol production, and understanding the components involved in cellulose synthesis is important for both fundamental and applied research. Despite the significance in the biofuel industry, the genes involved in sorghum cell wall biosynthesis, modification, and degradation have not been characterized. In this study, we have identified and characterized three allelic thick leaf mutants (thl1, thl2, and thl3). Bulked Segregant Analysis sequencing (BSAseq) showed that the causal mutation for the thl phenotype is in endo-1,4-β-glucanase gene (SbKOR1). Consistent with the causal gene function, the thl mutants showed decreased crystalline cellulose content in the stem tissues. The SbKOR1 function was characterized using Arabidopsis endo-1,4-β-glucanase gene mutant (rsw2-1). Complementation of Arabidopsis with SbKOR1 (native Arabidopsis promoter and overexpression by 35S promoter) restored the radial swelling phenotype of rsw2-1 mutant, proving that SbKOR1 functions as endo-1,4-β-glucanase. Overall, the present study has identified and characterized sorghum endo-1,4-β-glucanase gene function, laying the foundation for future research on cell wall biosynthesis and engineering of sorghum for biofuel production.

The Genetic Control of SEEDSTICK and LEUNIG-HOMOLOG in Seed and Fruit Development: New Insights into Cell Wall Control

Although much is known about seed and fruit development at the molecular level, many gaps remain in our understanding of how cell wall modifications can impact developmental processes in plants, as well as how biomechanical alterations influence seed and fruit growth. Mutants of Arabidopsis thaliana constitute an excellent tool to study the function of gene families devoted to cell wall biogenesis. We have characterized a collection of lines carrying mutations in representative cell wall-related genes for seed and fruit size developmental defects, as well as altered germination rates. We have linked these studies to cell wall composition and structure. Interestingly, we have found that disruption of genes involved in pectin maturation and hemicellulose deposition strongly influence germination dynamics. Finally, we focused on two transcriptional regulators, SEEDSTICK (STK) and LEUNIG-HOMOLOG (LUH), which positively regulate seed growth. Herein, we demonstrate that these factors regulate specific aspects of cell wall properties such as pectin distribution. We propose a model wherein changes in seed coat structure due to alterations in the xyloglucan-cellulose matrix deposition and pectin maturation are critical for organ growth and germination. The results demonstrate the importance of cell wall properties and remodeling of polysaccharides as major factors responsible for seed development.

Analysis of Elymus nutans seed coat development elucidates the genetic basis of metabolome and transcriptome underlying seed coat permeability characteristics

The seed coat takes an important function in the life cycle of plants, especially seed growth and development. It promotes the accumulation of nutrients inside the seed and protects the seed embryo from mechanical damage. Seed coat permeability is an important characteristic of seeds, which not only affects seed germination, but also hinders the detection of seed vigor by electrical conductivity (EC) method. This research aimed to elucidate the mechanism of seed coat permeability formation through metabolome and transcriptome analysis of Elymus nutans. We collected the samples at 8, 18, and 28 days post-anthesis (dpa), and conducted a seed inclusion exosmosis experiment and observed the seed coat permeability. Moreover, we analyzed the changes in the metabolome and transcriptome during different development stages. Here, taking 8 dpa as control, 252 upregulated and 157 downregulated differentially expressed metabolites (DEMs) were observed and 886 upregulated unigenes and 1170 downregulated unigenes were identified at 18 dpa, while 4907 upregulated unigenes and 8561 downregulated unigenes were identified at 28 dpa. Meanwhile, we observed the components of ABC transporters, the biosynthesis of unsaturated fatty acids, and phenylalanine metabolism pathways. The key metabolites and genes affecting seed coat permeability were thiamine and salicylic acid. Furthermore, there were 13 and 14 genes with correlation coefficients greater than 0.8 with two key metabolites, respectively, and the -log₂Fold Change- of these genes were greater than 1 at different development stages. Meanwhile, pathogenesis-related protein 1 and phenylalanine ammonia-lyase play an important role in regulating the formation of compounds. Our results outline a framework for understanding the development changes during seed growth of E. nutans and provide insights into the traits of seed coat permeability and supply a great significance value to seed production and quality evaluation.

Microtubule-associated IQD9 orchestrates cellulose patterning in seed mucilage

Arabidopsis seeds release large capsules of mucilaginous polysaccharides, which are shaped by an intricate network of cellulosic microfibrils. Cellulose synthase complexes are guided by the microtubule cytoskeleton, but it is unclear which proteins mediate this process in the seed coat epidermis. Using reverse genetics, we identified IQ67 DOMAIN 9 (IQD9) and KINESIN LIGHT CHAIN-RELATED 1 (KLCR1) as two highly expressed genes during seed development and comprehensively characterized their roles in cell wall polysaccharide biosynthesis. Mutations in IQD9 as well as in KLCR1 lead to compact mucilage capsules with aberrant cellulose distribution, which can be rescued by transgene complementation. IQD9 physically interacts with KLCR1 and localizes to cortical microtubules (MTs) to maintain their organization in seed coat epidermal (SCE) cells. IQD9 as well as a previously identified TONNEAU1 (TON1) RECRUITING MOTIF 4 (TRM4) protein act to maintain cellulose synthase velocity. Our results demonstrate that IQD9, KLCR1 and TRM4 are MT-associated proteins that are required for seed mucilage architecture. This study provides the first direct evidence that members of the IQD, KLCR and TRM families have overlapping roles in cell wall biosynthesis. Therefore, SCE cells provide an attractive system to further decipher the complex genetic regulation of polarized cellulose deposition.

Pectin methyltransferase QUASIMODO2 functions in the formation of seed coat mucilage in Arabidopsis

Pectin, cellulose, and hemicelluloses are major components of primary cell walls in plants. In addition to cell adhesion and expansion, pectin plays a central role in seed mucilage. Seed mucilage contains abundant pectic rhamnogalacturonan-I (RG-I) and lower amounts of homogalacturonan (HG), cellulose, and hemicelluloses. Previously, accumulated evidence has addressed the role of pectin RG-I in mucilage production and adherence. However, less is known about the function of pectin HG in seed coat mucilage formation. In this study, we analyzed a novel mutant, designated things fall apart2 (tfa2), which contains a mutation in HG methyltransferase QUASIMODO2 (QUA2). Etiolated tfa2 seedlings display short hypocotyls and adhesion defects similar to qua2 and tumorous shoot development2 (tsd2) alleles, and show seed mucilage defects. The diminished uronic acid content and methylesterification degree of HG in mutant seed mucilage indicate the role of HG in the formation of seed mucilage. Cellulosic rays in mutant mucilage are collapsed. The epidermal cells of seed coat in tfa2 and tsd2 display deformed columellae and reduced radial wall thickness. Under polyethylene glycol treatment, seeds from these three mutant alleles exhibit reduced germination rates. Together, these data emphasize the requirement of pectic HG biosynthesis for the synthesis of seed mucilage, and the functions of different pectin domains together with cellulose in regulating its formation, expansion, and release.

Receptor-like kinase HAESA-like 1 positively regulates seed longevity in Arabidopsis

Based on the phenotypic, physiological and transcriptomic analysis, receptor-like kinase HAESA-like 1 was demonstrated to positively affect seed longevity in Arabidopsis. Seed longevity is very important for both genetic resource conservation and crop production. Receptor-like kinases (RLKs) are widely involved in plant growth, development and stress responses. However, the role of most RLKs, especially in seed longevity, is largely unknown. In this study, we report that Arabidopsis HAESA-like 1 (AtHSL1) positively regulated seed longevity. Disruption of HSL1 significantly decreased the germination rate to 50% at 7 days after cold stratification (DAC), compared with that of the wild type (93.5% at 7 DAC), after accelerated aging treatment. Expression of the HSL1 gene in hsl1 basically restored the defective phenotype (86.3%), while HSL1-overexpressing lines (98.3%) displayed slower accelerated aging than WT (93.5%). GUS staining revealed HSL1 was highly expressed universally, especially in young seedlings, mature seeds and embryos of imbibed seeds, and its expression could be induced by accelerated aging. No difference in the dyeing color and area of mucilage were identified between WT and hsl1. The soluble pectin content also was not different, while the adherent pectin content was significantly increased in hsl1. Global transcriptomics revealed that disruption of HSL1 mainly downregulated genes involved in trehalose synthesis, nucleotide sugar metabolism and protection and repair mechanisms. Therefore, an increase in adherent pectin content and downregulation of genes involved in trehalose synthesis may be the main reasons for decreasing seed longevity owing to disruption of HSL1 in Arabidopsis. Our work provides valuable information for understanding the function and mechanism of a receptor-like kinase, AtHSL1, in seed longevity.

Mutation of CESA1 phosphorylation site influences pectin synthesis and methylesterification with a role in seed development

Cell wall biogenesis is required for the production of seeds of higher plants. However, little is known about regulatory mechanisms underlying cell wall biogenesis during seed formation. Here we show a role for the phosphorylation of Arabidopsis cellulose synthase 1 (AtCESA1) in modulating pectin synthesis and methylesterification in seed coat mucilage. A phosphor-null mutant of AtCESA1 on T166 (AtCESA1^T166A) was constructed and introduced into a null mutant of AtCESA1 (Atcesa1-1). The resulting transgenic lines showed a slight but significant decrease in cellulose contents in mature seeds. Defects in cellulosic ray architecture along with reduced levels of non-adherent and adherent mucilage were observed on the seeds of the AtCESA1^T166A mutant. Reduced mucilage pectin synthesis was also reflected by a decrease in the level of uronic acid. Meanwhile, an increase in the degree of pectin methylesterification was also observed in the seed coat mucilage of AtCESA1^T166A mutant. Change in seed development was further reflected by a delayed germination and about 50% increase in the accumulation of proanthocyanidins, which is known to bind pectin and inhibit seed germination as revealed by previous studies. Taken together, the results suggest a role of AtCESA1 phosphorylation on T166 in modulating mucilage pectin synthesis and methylesterification as well as cellulose synthesis with a role in seed development.

A new network containing MYB109-ZmCesA5 is involved in kernel development

MYB genes regulate several different aspects of metabolism and development. However, few studies have reported the involvement of MYBs-CesAs network in the regulation of maize kernel development. In this study, yeast one-hybrid (Y1H) assays and dual-luciferase reporter assays showed that ZmMYB109 activated the expression of ZmCesA5 by directly binding to its promoter. Real-time quantitative PCR (RT-qPCR) and transcriptome analyses showed that ZmMYB109 expression increased in ZmCesA5-OE kernels and decreased in ZmCesA5-KO kernels. Overexpression of ZmCesA5 produced heavier kernels, whereas loss of function of ZmCesA5 affected starch and sucrose metabolism, resulting in weight reduction of the maize kernels. Collectively, these findings suggest that a new network containing MYB109-ZmCesA5 is involved in kernel development.

Galactoglucomannan structure of Arabidopsis seed-coat mucilage in GDP-mannose synthesis impaired mutants

Cell-wall polysaccharides are synthesized from nucleotide sugars by glycosyltransferases. However, in what way the level of nucleotide sugars affects the structure of the polysaccharides is not entirely clear. guanosine diphosphate (GDP)-mannose (GDP-Man) is one of the major nucleotide sugars in plants and serves as a substrate in the synthesis of mannan polysaccharides. GDP-Man is synthesized from mannose 1-phosphate and GTP by a GDP-Man pyrophosphorylase, VITAMIN C DEFECTIVE1 (VTC1), which is positively regulated by the interacting protein KONJAC1 (KJC1) in Arabidopsis. Since seed-coat mucilage can serve as a model of the plant cell wall, we examined the influence of vtc1 and kjc1 mutations on the synthesis of mucilage galactoglucomannan. Sugar composition analysis showed that mannose content in adherent mucilage of kjc1 and vtc1 mutants was only 42% and 11% of the wild-type, respectively, indicating a drastic decrease of galactoglucomannan. On the other hand, structural analysis based on specific oligosaccharides released by endo-β-1,4-mannanase indicated that galactoglucomannan had a patterned glucomannan backbone consisting of alternating residues of glucose and mannose and the frequency of α-galactosyl branches was also similar to the wild type structure. These results suggest that the structure of mucilage galactoglucomannan is mainly determined by properties of glycosyltransferases rather than the availability of nucleotide sugars.

Cell wall formation pathways are differentially regulated in sugarcane contrasting genotypes associated with endophytic diazotrophic bacteria

Differences in cell wall components between two BNF-contrasting sugarcane genotypes might result from genetic variations particular to the genotype and from the efficiency in diazotrophic bacteria association. Sugarcane is a plant of the grass family (Poaceae) that is highly cultivated in Brazil, as an important energy resource. Commercial sugarcane genotypes may be successfully associated with beneficial endophytic nitrogen-fixing bacteria, which can influence several plant metabolic pathways, such as cell division and growth, synthesis of hormones, and defense compounds. In this study, we investigated how diazotrophic bacteria associated with sugarcane plants could be involved in the regulation of cell wall formation pathways. A molecular and structural characterization of the cell wall was compared between two genotypes of sugarcane with contrasting rates of Biological Nitrogen Fixation (BNF): SP70-1143 (high BNF) and Chunee (low BNF). Differentially expressed transcripts were identified in transcriptomes generated from SP70-1143 and Chunee. Expression profiles of cellulose and lignin genes, which were more expressed in SP70-1134, and callose genes, which were more expressed in Chunee, were validated by RT-qPCR and microscopic analysis of cell wall components in tissue sections. A similar expression profile in both BNF-contrasting genotypes was observed in naturally colonized plants and in plants inoculated with G. diazotrophicus. Cell walls of the high BNF genotype have a greater cellulose content, which might contribute to increase biomass. In parallel, callose was concentrated in the vascular tissues of the low BNF genotype and could possibly represent a barrier for an efficient bacterial colonization and dissemination in sugarcane tissues. Our data show a correlation between the gene profiles identified in the BNF-contrasting genotypes and a successful association with endophytic diazotrophic bacteria.

Differential Gene Expression of Brachypodium distachyon Roots Colonized by Gluconacetobacter diazotrophicus and the Role of BdCESA8 in the Colonization

Alternatives to synthetic nitrogen fertilizer are needed to reduce the costs of crop production and offset environmental damage. Nitrogen-fixing bacterium Gluconacetobacter diazotrophicus has been proposed as a possible biofertilizer for monocot crop production. However, the colonization of G. diazotrophicus in most monocot crops is limited and deep understanding of the response of host plants to G. diazotrophicus colonization is still lacking. In this study, the molecular response of the monocot plant model Brachypodium distachyon was studied during G. diazotrophicus root colonization. The gene expression profiles of B. distachyon root tissues colonized by G. diazotrophicus were generated via next-generation RNA sequencing, and investigated through gene ontology and metabolic pathway analysis. The RNA sequencing results indicated that Brachypodium is actively involved in G. diazotrophicus colonization via cell wall synthesis. Jasmonic acid, ethylene, gibberellin biosynthesis. nitrogen assimilation, and primary and secondary metabolite pathways are also modulated to accommodate and control the extent of G. diazotrophicus colonization. Cellulose synthesis is significantly downregulated during colonization. The loss of function mutant for Brachypodium cellulose synthase 8 (BdCESA8) showed decreased cellulose content in xylem and increased resistance to G. diazotrophicus colonization. This result suggested that the cellulose synthesis of the secondary cell wall is involved in G. diazotrophicus colonization. The results of this study provide insights for future research in regard to gene manipulation for efficient colonization of nitrogen-fixing bacteria in Brachypodium and monocot crops.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Sterol Glucosyltransferases Tailor Polysaccharide Accumulation in Arabidopsis Seed Coat Epidermal Cells

The conjugation of sterols with a Glc moiety is catalyzed by sterol glucosyltransferases (SGTs). A portion of the resulting steryl glucosides (SG) are then esterified with a long-chain fatty acid to form acyl-SG (ASG). SG and ASG are prevalent components of plant cellular membranes and influence their organization and functional properties. Mutant analysis had previously inferred that two Arabidopsis SGTs, UGT80A2 and UGT80B1/TT15, could have specialized roles in the production of SG in seeds, despite an overlap in their enzymatic activity. Here, we establish new roles for both enzymes in the accumulation of polysaccharides in seed coat epidermal cells (SCEs). The rhamnogalacturonan-I (RG-I) content of the inner layer of seed mucilage was higher in ugt80A2, whereas RG-I accumulation was lower in mutants of UGT80B1, with double mutant phenotypes indicating that UGT80A2 acts independently from UGT80B1. In contrast, an additive phenotype was observed in double mutants for increased galactoglucomannan (GGM) content. Double mutants also exhibited increased polymer density within the inner mucilage layer. In contrast, cell wall defects were only observed in mutants defective for UGT80B1, while more mucilage cellulose was only observed when UGT80A2 was mutated. The generation of a range of phenotypic effects, simultaneously within a single cell type, demonstrates that the adjustment of the SG and ASG composition of cellular membranes by UGT80A2 and UGT80B1 tailors polysaccharide accumulation in Arabidopsis seeds.

Conserved, divergent and heterochronic gene expression during Brachypodium and Arabidopsis embryo development

KEY MESSAGE

Developmental and transcriptomic analysis of Brachypodium embryogenesis and comparison with Arabidopsis identifies conserved and divergent phases of embryogenesis and reveals widespread heterochrony of developmental gene expression. Embryogenesis, transforming the zygote into the mature embryo, represents a fundamental process for all flowering plants. Current knowledge of cell specification and differentiation during plant embryogenesis is largely based on studies of the dicot model plant Arabidopsis thaliana. However, the major crops are monocots and the transcriptional programs associated with the differentiation processes during embryogenesis in this clade were largely unknown. Here, we combined analysis of cell division patterns with development of a temporal transcriptomic resource during embryogenesis of the monocot model plant Brachypodium distachyon. We found that early divisions of the Brachypodium embryo were highly regular, while later stages were marked by less stereotypic patterns. Comparative transcriptomic analysis between Brachypodium and Arabidopsis revealed that early and late embryogenesis shared a common transcriptional program, whereas mid-embryogenesis was divergent between species. Analysis of orthology groups revealed widespread heterochronic expression of potential developmental regulators between the species. Interestingly, Brachypodium genes tend to be expressed at earlier stages than Arabidopsis counterparts, which suggests that embryo patterning may occur early during Brachypodium embryogenesis. Detailed investigation of auxin-related genes shows that the capacity to synthesize, transport and respond to auxin is established early in the embryo. However, while early PIN1 polarity could be confirmed, it is unclear if an active response is mounted. This study presents a resource for studying Brachypodium and grass embryogenesis and shows that divergent angiosperms share a conserved genetic program that is marked by heterochronic gene expression.

Two β-glucuronosyltransferases involved in the biosynthesis of type II arabinogalactans function in mucilage polysaccharide matrix organization in Arabidopsis thaliana

BACKGROUND

Arabinogalactan-proteins (AGPs) are heavily glycosylated with type II arabinogalactan (AG) polysaccharides attached to hydroxyproline residues in their protein backbone. Type II AGs are necessary for plant growth and critically important for the establishment of normal cellular functions. Despite the importance of type II AGs in plant development, our understanding of the underlying role of these glycans/sugar residues in mucilage formation and seed coat epidermal cell development is poorly understood and far from complete. One such sugar residue is the glucuronic acid residues of AGPs that are transferred onto AGP glycans by the action of β-glucuronosyltransferase genes/enzymes.

RESULTS

Here, we have characterized two β-glucuronosyltransferase genes, GLCAT14A and GLCAT14C, that are involved in the transfer of β-glucuronic acid (GlcA) to type II AGs. Using a reverse genetics approach, we observed that glcat14a-1 mutants displayed subtle alterations in mucilage pectin homogalacturonan (HG) compared to wild type (WT), while glcat14a-1glcat14c-1 mutants displayed much more severe mucilage phenotypes, including loss of adherent mucilage and significant alterations in cellulose ray formation and seed coat morphology. Monosaccharide composition analysis showed significant alterations in the sugar amounts of glcat14a-1glcat14c-1 mutants relative to WT in the adherent and non-adherent seed mucilage. Also, a reduction in total mucilage content was observed in glcat14a-1glcat14c-1 mutants relative to WT. In addition, glcat14a-1glcat14c-1 mutants showed defects in pectin formation, calcium content and the degree of pectin methyl-esterification (DM) as well as reductions in crystalline cellulose content and seed size.

CONCLUSIONS

These results raise important questions regarding cell wall polymer interactions and organization during mucilage formation. We propose that the enzymatic activities of GLCAT14A and GLCAT14C play partially redundant roles and are required for the organization of the mucilage matrix and seed size in Arabidopsis thaliana. This work brings us a step closer towards identifying potential gene targets for engineering plant cell walls for industrial applications.

Seed coat mucilages: Structural, functional/bioactive properties, and genetic information

Seed coat mucilages are mainly polysaccharides covering the outer layer of the seeds to facilitate seed hydration and germination, thereby improving seedling emergence and reducing seedling mortality. Four types of polysaccharides are found in mucilages including xylan, pectin, glucomannan, and cellulose. Recently, mucilages from flaxseed, yellow mustard seed, chia seed, and so on, have been used extensively in the areas of food, pharmaceutical, and cosmetics contributing to stability, texture, and appearance. This review, for the first time, addresses the similarities and differences in physicochemical properties, molecular structure, and functional/bioactive properties of mucilages among different sources; highlights their structure and function relationships; and systematically summarizes the related genetic information, aiming with the intent to explore the potential functions thereby extending their future industrial applications.

HOMEODOMAIN GLABROUS2 regulates cellulose biosynthesis in seed coat mucilage by activating CELLULOSE SYNTHASE5

Numerous proteins involved in cellulose biosynthesis and assembly have been functionally characterized. Nevertheless, we have a limited understanding of the mechanisms underlying the transcriptional regulation of the genes that encode these proteins. Here, we report that HOMEODOMAIN GLABROUS2 (HDG2), a Homeobox-Leucine Zipper IV transcription factor, regulates cellulose biosynthesis in Arabidopsis (Arabidopsis thaliana) seed coat mucilage. HDG2 is a transcriptional activator with the transactivation domain located within its Leucine-Zipper domain. Transcripts of HDG2 were detected specifically in seed coat epidermal cells with peak expression at 10 d postanthesis. Disruptions of HDG2 led to seed coat mucilage with aberrant morphology due to a reduction in its crystalline cellulose content. Electrophoretic mobility shift and yeast one-hybrid assays, together with chromatin immunoprecipitation and quantitative PCR, provided evidence that HDG2 directly activates CELLULOSE SYNTHASE5 (CESA5) expression by binding to the L1-box cis-acting element in its promoter. Overexpression of CESA5 partially rescued the mucilage defects of hdg2-3. Together, our data suggest that HDG2 directly activates CESA5 expression and thus is a positive regulator of cellulose biosynthesis in seed coat mucilage.

Seed hemicelluloses tailor mucilage properties and salt tolerance

While Arabidopsis seed coat epidermal cells have become an excellent genetic system to study the biosynthesis and structural roles of various cell wall polymers, the physiological function of the secreted mucilaginous polysaccharides remains ambiguous. Seed mucilage is shaped by two distinct classes of highly substituted hemicelluloses along with cellulose and structural proteins, but their interplay has not been explored. We deciphered the functions of four distinct classes of cell wall polymers by generating a series of double mutants with defects in heteromannan, xylan, cellulose, or the arabinogalactan protein SALT-OVERLY SENSITIVE 5 (SOS5), and evaluating their impact on mucilage architecture and seed germination during salt stress. We discovered that muci10 seeds, lacking heteromannan branches, had elevated tolerance to salt stress, while heteromannan elongation mutants exhibited reduced germination in calcium chloride (CaCl₂ ). By contrast, xylan made by MUCILAGE-RELATED21 (MUCI21) was found to be required for the adherence of mucilage pectin to microfibrils made by CELLULOSE SYNTHASE5 (CESA5) as well as to a SOS5-mediated network. Our results indicate that the substitution of xylan and glucomannan in seeds can fine-tune mucilage adherence and salt tolerance, respectively. The study of germinating seeds can thus provide insights into the synthesis, modification and function of complex glycans.

Structural and quantitative changes of starch in seed of Trigonella persica during germination

Starch, the most abundant carbohydrate reserve in plant seeds, is synthesized and stored in the cotyledons of some plants. In seeds of Trigonella persica (Fabaceae), starch appears during germination and forms granules that are composed of amylose and/or amylopectin. In this study, both quantitative and qualitative traits of starch were evaluated during the germination of T. persica seed. The quantitative assay and microscopic imaging showed that there was a low-amylose starch in the cotyledons and its amount reached the highest at 48 h after imbibition (HAI). According to the SEM images of hydrogels, FT-IR spectra, and analysis of extracted starches, the purity of this starch was decreased during germination. So that the extracted starch from cotyledons of non-germinated seeds (ESCN), with the highest purity and the lowest protein content, was different from all the extracted starches. Therefore, it seems that ESCN was an amylopectin rich type starch, which had low amylose to amylopectin ratio and as waxy starch typically, it can have a cohesive and gummy texture compared with ordinary starch.

Genome-wide characterization of the cellulose synthase gene superfamily in Pyrus bretschneideri and reveal its potential role in stone cell formation

Members of the cellulose synthase (CesA) and cellulose synthase-like (Csl) families from the cellulose synthase gene superfamily participate in cellulose and hemicellulose synthesis in the plasma membrane. The members of this superfamily are vital for cell wall construction during plant growth and development. However, little is known about their function in pear fruit, a model for Rosaceae species and for fleshy fruit development. In our research, a total of 36 CesA/Csl family members were identified from the pear and were grouped into six subfamilies (CesA, CslB, CslC, CslD, CslE, and CslG) according to phylogenetic relationships. We performed a protein sequence physicochemical analysis, phylogenetic tree construction, a gene structure, a conserved domain, and chromosomal localization analysis. The results indicated that most of the CesA/Csl genes from pear are closely related to genes in Arabidopsis, but these families have unique characteristics in terms of their gene structure, chromosomal localization, phylogeny, and deduced protein sequences, suggesting that they have evolved through different processes. Tissue expression analysis results showed that most of the CesA/Csl genes were constitutively expressed at different levels in different organs. Furthermore, the expression levels of four genes (Pbr032894.2, Pbr016107.1, Pbr00518.1, and Pbr034218.1) tended to first increase and then decrease during fruit development, implying that these four genes may be involved in the development of stone cells of pear fruit. Our results may help elucidate the evolutionary history and functional differences of the CesA/Csl genes in pear and lay a foundation for further investigation of the CesA/Csl genes in pear and other Rosaceae species.

The N-terminal zinc finger of CELLULOSE SYNTHASE6 is critical in defining its functional properties by determining the level of homodimerization in Arabidopsis

Primary cell wall cellulose is synthesized by the cellulose synthase complex (CSC) containing CELLULOSE SYNTHASE1 (CESA1), CESA3 and one of four CESA6-like proteins in Arabidopsis. It has been proposed that the CESA6-like proteins occupy the same position in the CSC, but their underlying selection mechanism remains unclear. We produced a chimeric CESA5 by replacing its N-terminal zinc finger with its CESA6 counterpart to investigate the consequences for its homodimerization, a crucial step in forming higher-order structures during assembly of the CSC. We found that the mutant phenotypes of prc1-1, a cesa6 null mutant, were rescued by the chimeric CESA5, and became comparable to the wild type (WT) and prc1-1 complemented by WT CESA6 in regard to plant growth, cellulose content, cellulose microfibril organization, CSC dynamics and subcellular localization. Bimolecular fluorescence complementation assays were employed to evaluate pairwise interactions between the N-terminal regions of CESA1, CESA3, CESA5, CESA6 and the chimeric CESA5. We verified that the chimeric CESA5 explicitly interacted with all the other CESA partners, comparable to CESA6, whereas interaction between CESA5 with itself was significantly weaker than that of all other CESA pairs. Our findings suggest that the homodimerization of CESA6 through its N-terminal zinc finger is critical in defining its functional properties, and possibly determines its intrinsic roles in facilitating higher-order structures in CSCs.

Seed-specific down-regulation of Arabidopsis CELLULOSE SYNTHASE 1 or 9 reduces seed cellulose content and differentially affects carbon partitioning

Seed-specific down-regulation of AtCESA1 and AtCESA9, which encode cellulose synthase subunits, differentially affects seed storage compound accumulation in Arabidopsis. High amounts of cellulose can negatively affect crop seed quality, and, therefore, diverting carbon partitioning from cellulose to oil, protein and/or starch via molecular breeding may improve seed quality. To determine the effect of seed cellulose content reduction on levels of storage compounds, Arabidopsis thaliana CELLULOSE SYNTHASE1 (AtCESA1) and AtCESA9 genes, which both encode cellulose synthase subunits, were individually down-regulated using seed-specific intron-spliced hairpin RNA (hpRNAi) constructs. The selected seed-specific AtCESA1 and AtCESA9 Arabidopsis RNAi lines displayed reduced cellulose contents in seeds, and exhibited no obvious visual phenotypic growth defects with the exception of a minor effect on early root development in AtCESA1 RNAi seedlings and early hypocotyl elongation in the dark in both types of RNAi line. The seed-specific down-regulation of AtCESA9 resulted in a reduction in seed weight compared to empty vector controls, which was not observed in AtCESA1 RNAi lines. In terms of effects on carbon partitioning, AtCESA1 and AtCESA9 RNAi lines exhibited distinct effects. The down-regulation of AtCESA1 led to a ~ 3% relative increase in seed protein content (P = 0.04) and a ~ 3% relative decrease in oil content (P = 0.02), but caused no alteration in soluble glucose levels. On the contrary, AtCESA9 RNAi lines did not display a significant reduction in seed oil, protein or soluble glucose content. Taken together, our results indicate that the seed-specific down-regulation of AtCESA1 causes alterations in seed storage compound accumulation, while the effect of AtCESA9 on carbon partitioning is absent or minor in Arabidopsis.

Transcription Factors BLH2 and BLH4 Regulate Demethylesterification of Homogalacturonan in Seed Mucilage

The polysaccharide pectin is a major component of the plant cell wall. The pectic glycan homogalacturonan (HG) is a proportionally small but important component of a specialized seed cell wall called mucilage. HG is synthesized in a highly methylesterified form, and, following secretion, is de-methylesterified by pectin methylesterases (PMEs). The degree of methylesterification of HG determines the structural and functional properties of pectin, but how methylesterification is regulated remains largely unknown. Here, we identified two BEL1-Like homeodomain (BLH) transcription factors, BLH2 and BLH4, as positive regulators of HG de-methylesterification in Arabidopsis (Arabidopsis thaliana) seed coat mucilage. BLH2 and BLH4 were significantly expressed in mucilage secretory cells during seed mucilage production. BLH2 and BLH4 single mutants exhibited no obvious mucilage phenotype, but the blh2 blh4 double mutant displayed significantly reduced mucilage adherence to the seed. Reduced mucilage adherence in blh2 blh4 was caused by decreased PME activity in the seed coat, which increased the degree of methylesterification of HG in mucilage. The expression of several PME metabolism-related genes, including PME58, PECTIN METHYLESTERASE INHIBITOR6, SEEDSTICK, and MYB52 was significantly altered in blh2 blh4 seeds. BLH2 and BLH4 directly activated PME58 expression by binding to its TGACAGGT cis-element. Moreover, pme58 mutants exhibited reduced mucilage adherence similar to that of blh2 blh4, and the blh2 blh4 pme58 triple mutant exhibited no additional mucilage adherence defects. Furthermore, overexpression of PME58 in blh2 blh4 rescued the mucilage adherence defect. Together, these results demonstrate that BLH2 and BLH4 redundantly regulate de-methylesterification of HG in seed mucilage by directly activating PME58.

Dynamic Construction, Perception, and Remodeling of Plant Cell Walls

Plant cell walls are dynamic structures that are synthesized by plants to provide durable coverings for the delicate cells they encase. They are made of polysaccharides, proteins, and other biomolecules and have evolved to withstand large amounts of physical force and to resist external attack by herbivores and pathogens but can in many cases expand, contract, and undergo controlled degradation and reconstruction to facilitate developmental transitions and regulate plant physiology and reproduction. Recent advances in genetics, microscopy, biochemistry, structural biology, and physical characterization methods have revealed a diverse set of mechanisms by which plant cells dynamically monitor and regulate the composition and architecture of their cell walls, but much remains to be discovered about how the nanoscale assembly of these remarkable structures underpins the majestic forms and vital ecological functions achieved by plants.

Increase in Cell Wall Thickening and Biomass Production by Overexpression of PmCesA2 in Poplar

Cellulose, the most abundant constituent material of the plant cell walls, is a major structural component of plant biomass. Manipulating cellulose synthesis (CesA) genes by genetic engineering technology, to increase cellulose production may thus offer novel opportunities for plant growth and development. To investigate this, here we produced transgenic "Populus 895 plants" overexpressing the cellulose synthase (CesA2) gene derived from Pinus massoniana under the control of constitutive 35S promoter, via Agrobacterium-mediated transformation. Relative expression levels of PmCesA2 were functionally characterized in poplar hybrid clone "Nanlin895" (Populus deltoides × Populus euramericana). The results demonstrated the transgenic lines showed enhanced growth performance with increased biomass production than did the untransformed controls. It is noteworthy that the overexpression of PmCesA2 in poplar led to an altered cell wall polysaccharide composition, which resulted in the thickening of the secondary cell wall and xylem width under scanning electron microscopy. Consequently, the cellulose and lignin content were increased. Hence, this study suggests that overexpression of PmCesA2 could be used as a potential candidate gene to enhance cellulose synthesis and biomass accumulation in genetically engineered trees.

Analysis of Monosaccharides from Arabidopsis Seed Mucilageand Whole Seeds Using HPAEC-PAD

Arabidopsis seed coat epidermal cells deposit a significant quantity of mucilage, composed of the cell wall components pectin, hemicellulose, and cellulose, into the apoplast during development. When mature seeds are hydrated, mucilage extrudes to form a gelatinous capsule around the seed. Determining the monosaccharide composition of both extruded mucilage and whole seeds is an essential technique for characterizing seed coat developmental processes and mutants with altered mucilage composition. This protocol covers growth of plants to produce seeds suitable for analysis, extraction of extruded mucilage using water and sodium carbonate (used for mutants with impaired mucilage release), and extraction of alcohol insoluble residue (AIR) from whole seeds. The prepared polysaccharides are then hydrolyzed using sulfuric acid, which hydrolyses all polysaccharides including cellulose. Sensitive and reproducible quantification of the resulting monosaccharides is achieved using high-performance anion exchange chromatography coupled with pulsed amperometric detection (HPAEC-PAD).

Natural Variation Reveals a Key Role for Rhamnogalacturonan I in Seed Outer Mucilage and Underlying Genes

On imbibition, Arabidopsis (Arabidopsis thaliana) seeds release polysaccharides from their epidermal cells that form a two-layered hydrogel, termed mucilage. Analysis of a publicly available data set of outer seed mucilage traits of over 300 accessions showed little natural variation in composition. This mucilage is almost exclusively made up of rhamnogalacturonan I (RGI), highlighting the importance of this pectin for outer mucilage function. In a genome-wide association study, observed variations in polymer amount and macromolecular characteristics were linked to several genome polymorphisms, indicating the complexity of their genetic regulation. Natural variants with high molar mass were associated with a gene encoding a putative glycosyltransferase called MUCILAGE-RELATED70 (MUCI70). muci70 insertion mutants produced many short RGI polymers that were highly substituted with xylan, confirming that polymorphism in this gene can affect RGI polymer size. A second gene encoding a putative copper amine oxidase of clade 1a (CuAOα1) was associated with natural variation in the amount of RGI present in the outer mucilage layer; cuaoα1 mutants validated its role in pectin production. As the mutant phenotype is unique, with RGI production only impaired for outer mucilage, this indicates that CuAOα1 contributes to a further mechanism controlling mucilage synthesis.

ECERIFERUM11/C-TERMINAL DOMAIN PHOSPHATASE-LIKE2 Affects Secretory Trafficking

Secretory trafficking is highly conserved in all eukaryotic cells and is required for secretion of proteins as well as extracellular matrix components. In plants, the export of cuticular waxes and various cell wall components relies on secretory trafficking, but the molecular mechanisms underlying their secretion are not well understood. In this study, we characterize the Arabidopsis (Arabidopsis thaliana) dwarf eceriferum11 (cer11) mutant and we show that it exhibits reduced stem cuticular wax deposition, aberrant seed coat mucilage extrusion, and delayed secondary cell wall columella formation, as well as a block in secretory GFP trafficking. Cloning of the CER11 gene revealed that it encodes a C-TERMINAL DOMAIN PHOSPHATASE-LIKE2 (CPL2) protein. Thus, secretory trafficking in plant cells in general, and secretion of extracellular matrix constituents in developing epidermal cells in particular, involves a dephosphorylation step catalyzed by CER11/CPL2.

The Photoperiodic Flowering Time Regulator FKF1 Negatively Regulates Cellulose Biosynthesis

Cellulose synthesis is precisely regulated by internal and external cues, and emerging evidence suggests that light regulates cellulose biosynthesis through specific light receptors. Recently, the blue light receptor CRYPTOCHROME 1 (CRY1) was shown to positively regulate secondary cell wall biosynthesis in Arabidopsis (Arabidopsis thaliana). Here, we characterize the role of FLAVIN-BINDING KELCH REPEAT, F-BOX 1 (FKF1), another blue light receptor and well-known photoperiodic flowering time regulator, in cellulose biosynthesis. A phenotype suppression screen using a cellulose deficient mutant cesa1aegeus,cesa3ixr1-2 (c1,c3), which carries nonlethal point mutations in CELLULOSE SYNTHASE A 1 (CESA1) and CESA3, resulted in identification of the phenotype-restoring large leaf (llf) mutant. Next-generation mapping using the whole genome resequencing method identified the llf locus as FKF1 FKF1 was confirmed as the causal gene through observation of the llf phenotype in an independent triple mutant c1,c3,fkf1-t carrying a FKF1 T-DNA insertion mutant. Moreover, overexpression of FKF1 in llf plants restored the c1,c3 phenotype. The fkf1 mutants showed significant increases in cellulose content and CESA gene expression compared with that in wild-type Columbia-0 plants, suggesting a negative role of FKF1 in cellulose biosynthesis. Using genetic, molecular, and phenocopy and biochemical evidence, we have firmly established the role of FKF1 in regulation of cellulose biosynthesis. In addition, CESA expression analysis showed that diurnal expression patterns of CESAs are FKF1 independent, whereas their circadian expression patterns are FKF1 dependent. Overall, our work establishes a role of FKF1 in the regulation of cell wall biosynthesis in Arabidopsis.

Cytological Approaches Combined With Chemical Analysis Reveals the Layered Nature of Flax Mucilage

The external seed coat cell layer of certain species is specialized in the production and extrusion of a polysaccharide matrix called mucilage. Variations in the content of the released mucilage have been mainly associated with genetically regulated physiological modifications. Understanding the mucilage extrusion process in crop species is of importance to gain deeper insight into the complex cell wall biosynthesis and dynamics. In this study, we took advantage of the varying polysaccharide composition and the size of the flax mucilage secretory cells (MSCs) to study mucilage composition and extrusion in this species of agricultural interest. We demonstrate herein that flax MSCs are structured in four superimposed layers and that rhamnogalacturonans I (RG I) are firstly synthesized, in the upper face, preceding arabinoxylan and glucan synthesis in MSC lower layers. Our results also reveal that the flax mucilage release originates from inside MSC, between the upper and deeper layers, the latter collaborating to trigger polysaccharide expansion, radial cell wall breaking and mucilage extrusion in a peeling fashion. Here, we provide evidence that the layer organization and polysaccharide composition of the MSCs regulate the mucilage release efficiency like a peeling mechanism. Finally, we propose that flax MSCs may represent an excellent model for further investigations of mucilage biosynthesis and its release.

Primary wall cellulose synthase regulates shoot apical meristem mechanics and growth

How organisms attain their specific shapes and modify their growth patterns in response to environmental and chemical signals has been the subject of many investigations. Plant cells are at high turgor pressure and are surrounded by a rigid yet flexible cell wall, which is the primary determinant of plant growth and morphogenesis. Cellulose microfibrils, synthesized by plasma membrane-localized cellulose synthase complexes, are major tension-bearing components of the cell wall that mediate directional growth. Despite advances in understanding the genetic and biophysical regulation of morphogenesis, direct studies of cellulose biosynthesis and its impact on morphogenesis of different cell and tissue types are largely lacking. In this study, we took advantage of mutants of three primary cellulose synthase (CESA) genes that are involved in primary wall cellulose synthesis. Using field emission scanning electron microscopy, live cell imaging and biophysical measurements, we aimed to understand how the primary wall CESA complex acts during shoot apical meristem development. Our results indicate that cellulose biosynthesis impacts the mechanics and growth of the shoot apical meristem.

RUBY, a Putative Galactose Oxidase, Influences Pectin Properties and Promotes Cell-To-Cell Adhesion in the Seed Coat Epidermis of Arabidopsis

Cell-to-cell adhesion is essential for establishment of multicellularity. In plants, such adhesion is mediated through a middle lamella composed primarily of pectic polysaccharides. The molecular interactions that influence cell-to-cell adhesion are not fully understood. We have used Arabidopsis (Arabidopsis thaliana) seed coat mucilage as a model system to investigate interactions between cell wall carbohydrates. Using a forward-genetic approach, we have discovered a gene, RUBY PARTICLES IN MUCILAGE (RUBY), encoding a protein that is annotated as a member of the Auxiliary Activity 5 (AA5) family of Carbohydrate-Active Enzymes (Gal/glyoxal oxidases) and is secreted to the apoplast late in the differentiation of seed coat epidermal cells. We show that RUBY is required for the Gal oxidase activity of intact seeds; the oxidation of Gal in side-chains of rhamnogalacturonan-I (RG-I) present in mucilage-modified2 (mum2) mucilage, but not in wild-type mucilage; the retention of branched RG-I in the seed following extrusion; and the enhancement of cell-to-cell adhesion in the seed coat epidermis. These data support the hypothesis that RUBY is a Gal oxidase that strengthens pectin cohesion within the middle lamella, and possibly the mucilage of wild-type seed coat epidermal cells, through oxidation of RG-I Gal side-chains.

The Regulation of Cellulose Biosynthesis in Plants

Cell walls define the shape of plant cells, controlling the extent and orientation of cell elongation, and hence organ growth. The main load-bearing component of plant cell walls is cellulose, and how plants regulate its biosynthesis during development and in response to various environmental perturbations is a central question in plant biology. Cellulose is synthesized by cellulose synthase (CESA) complexes (CSCs) that are assembled in the Golgi apparatus and then delivered to the plasma membrane (PM), where they actively synthesize cellulose. CSCs travel along cortical microtubule paths that define the orientation of synthesis of the cellulose microfibrils. CSCs recycle between the PM and various intracellular compartments, and this trafficking plays an important role in determining the level of cellulose synthesized. In this review, we summarize recent findings in CESA complex organization, CESA posttranslational modifications and trafficking, and other components that interact with CESAs. We also discuss cell wall integrity maintenance, with a focus on how this impacts cellulose biosynthesis.

TRM4 is essential for cellulose deposition in Arabidopsis seed mucilage by maintaining cortical microtubule organization and interacting with CESA3

The differentiation of the seed coat epidermal (SCE) cells in Arabidopsis thaliana leads to the production of a large amount of pectin-rich mucilage and a thick cellulosic secondary cell wall. The mechanisms by which cortical microtubules are involved in the formation of these pectinaceous and cellulosic cell walls are still largely unknown. Using a reverse genetic approach, we found that TONNEAU1 (TON1) recruiting motif 4 (TRM4) is implicated in cortical microtubule organization in SCE cells, and functions as a novel player in the establishment of mucilage structure. TRM4 is preferentially accumulated in the SCE cells at the stage of mucilage biosynthesis. The loss of TRM4 results in compact seed mucilage capsules, aberrant mucilage cellulosic structure, short cellulosic rays and disorganized cellulose microfibrils in mucilage. The defects could be rescued by transgene complementation of trm4 alleles. Probably, this is a consequence of a disrupted organization of cortical microtubules, observed using fluorescently tagged tubulin proteins in trm4 SCE cells. Furthermore, TRM4 proteins co-aligned with microtubules and interacted directly with CELLULOSE SYNTHASE 3 in two independent assays. Together, the results indicate that TRM4 is essential for microtubule array organization and therefore correct cellulose orientation in the SCE cells, as well as the establishment of the subsequent mucilage architecture.

MuSeeQ, a novel supervised image analysis tool for the simultaneous phenotyping of the soluble mucilage and seed morphometric parameters

BACKGROUND

The mucilage is a model to study the polysaccharide biosynthesis since it is produced in large amounts and composed of complex polymers. In addition, it is of great economic interest for its technical and nutritional value. A fast method for phenotyping the released mucilage and the seed morphometric parameters will be useful for fundamental, food, pharmaceutical and breeding researches. Current strategies to phenotype soluble mucilage are restricted to visual evaluations or are highly time-consuming.

RESULTS

Here, we developed a high-throughput phenotyping method for the simultaneous measurement of the soluble mucilage content released on a gel and the seed morphometric parameters. Within this context, we combined a biochemical assay and an open-source computer-aided image analysis tool, MuSeeQ. The biochemical assay consists in sowing seeds on an agarose medium containing the dye toluidine blue O, which specifically stains the mucilage once it is released on the gel. The second part of MuSeeQ is a macro developed in ImageJ allowing to quickly extract and analyse 11 morphometric data of seeds and their respective released mucilages. As an example, MuSeeQ was applied on a flax recombinant inbred lines population (previously screened for fatty acids content.) and revealed significant correlations between the soluble mucilage shape and the concentration of some fatty acids, e.g. C16:0 and C18:2. Other fatty acids were also found to correlate with the seed shape parameters, e.g. C18:0 and C18:2. MuSeeQ was then showed to be used for the analysis of other myxospermous species, including Arabidopsis thaliana and Camelina sativa.

CONCLUSIONS

MuSeeQ is a low-cost and user-friendly method which may be used by breeders and researchers for phenotyping simultaneously seeds of specific cultivars, natural variants or mutants and their respective soluble mucilage area released on a gel. The script of MuSeeQ and video tutorials are freely available at http://MuSeeQ.free.fr.

Role for Arabidopsis PLC7 in Stomatal Movement, Seed Mucilage Attachment, and Leaf Serration

Phospholipase C (PLC) has been suggested to play important roles in plant stress and development. To increase our understanding of PLC signaling in plants, we have started to analyze knock-out (KO), knock-down (KD) and overexpression mutants of Arabidopsis thaliana, which contains nine PLCs. Earlier, we characterized PLC2, PLC3 and PLC5. Here, the role of PLC7 is functionally addressed. Promoter-GUS analyses revealed that PLC7 is specifically expressed in the phloem of roots, leaves and flowers, and is also present in trichomes and hydathodes. Two T-DNA insertion mutants were obtained, i.e., plc7-3 being a KO- and plc7-4 a KD line. In contrast to earlier characterized phloem-expressed PLC mutants, i.e., plc3 and plc5, no defects in primary- or lateral root development were found for plc7 mutants. Like plc3 mutants, they were less sensitive to ABA during stomatal closure. Double-knockout plc3 plc7 lines were lethal, but plc5 plc7 (plc5/7) double mutants were viable, and revealed several new phenotypes, not observed earlier in the single mutants. These include a defect in seed mucilage, enhanced leaf serration, and an increased tolerance to drought. Overexpression of PLC7 enhanced drought tolerance too, similar to what was earlier found for PLC3-and PLC5 overexpression. In vivo 32Pi-labeling of seedlings and treatment with sorbitol to mimic drought stress, revealed stronger PIP2 responses in both drought-tolerant plc5/7 and PLC7-OE mutants. Together, these results show novel functions for PLC in plant stress and development. Potential molecular mechanisms are discussed.

Identification of Key Enzymes for Pectin Synthesis in Seed Mucilage

Pectin is a vital component of the plant cell wall and provides the molecular glue that maintains cell-cell adhesion, among other functions. As the most complex wall polysaccharide, pectin is composed of several covalently linked domains, such as homogalacturonan (HG) and rhamnogalacturonan I (RG I). Pectin has widespread uses in the food industry and has emerging biomedical applications, but its synthesis remains poorly understood. For instance, the enzymes that catalyze RG I elongation remain unknown. Recently, a coexpression- and sequence-based MUCILAGE-RELATED (MUCI) reverse genetic screen uncovered hemicellulose biosynthetic enzymes in the Arabidopsis (Arabidopsis thaliana) seed coat. Here, we use an extension of this strategy to identify MUCI70 as the founding member of a glycosyltransferase family essential for the accumulation of seed mucilage, a gelatinous wall rich in unbranched RG I. Detailed biochemical and histological characterization of two muci70 mutants and two galacturonosyltransferase11 (gaut11) mutants identified MUCI70 and GAUT11 as required for two distinct RG I domains in seed mucilage. We demonstrate that, unlike MUCI70, GAUT11 catalyzes HG elongation in vitro and, thus, likely is required for the synthesis of an HG region important for RG I elongation. Analysis of a muci70 gaut11 double mutant confirmed that MUCI70 and GAUT11 are indispensable for the production and release of the bulk of mucilage RG I and for shaping the surface morphology of seeds. In addition, we uncover relationships between pectin and hemicelluloses and show that xylan is essential for the elongation of at least one RG I domain.

GhHUB2, a ubiquitin ligase, is involved in cotton fiber development via the ubiquitin-26S proteasome pathway

Cotton fibers, which are extremely elongated single cells of epidermal seed trichomes and have highly thickened cell walls, constitute the most important natural textile material worldwide. However, the regulation of fiber development is not well understood. Here, we report that GhHUB2, a functional homolog of AtHUB2, controls fiber elongation and secondary cell wall (SCW) deposition. GhHUB2 is ubiquitously expressed, including within fibers. Overexpression of GhHUB2 in cotton increased fiber length and SCW thickness, while RNAi knockdown of GhHUB2 resulted in shortened fibers and thinner cell walls. We found that GhHUB2 interacted with GhKNL1, a transcriptional repressor predominantly expressed in developing fibers, and that GhHUB2 ubiquitinated and degraded GhKNL1 via the ubiquitin-26S proteasome pathway. GhHUB2 negatively regulated GhKNL1 protein levels and lead to the disinhibition of genes such as GhXTH1, Gh1,3-β-G, GhCesA4, GhAGP4, GhCTL1, and GhCOBL4, thus promoting fiber elongation and enhancing SCW biosynthesis. We found that GhREV-08, a transcription factor that participates in SCW deposition and auxin signaling pathway, was a direct target of GhKNL1. In conclusion, our study uncovers a novel function of HUB2 in plants in addition to its monoubiquitination of H2B. Moreover, we provide evidence for control of the fiber development by the ubiquitin-26S proteasome pathway.

Emerging Functions for Cell Wall Polysaccharides Accumulated during Eudicot Seed Development

The formation of seeds is a reproductive strategy in higher plants that enables the dispersal of offspring through time and space. Eudicot seeds comprise three main components, the embryo, the endosperm and the seed coat, where the coordinated development of each is important for the correct formation of the mature seed. In addition, the seed coat protects the quiescent progeny and can provide transport mechanisms. A key underlying process in the production of seed tissues is the formation of an extracellular matrix termed the cell wall, which is well known for its essential function in cytokinesis, directional growth and morphogenesis. The cell wall is composed of a macromolecular network of polymers where the major component is polysaccharides. The attributes of polysaccharides differ with their composition and charge, which enables dynamic remodeling of the mechanical and physical properties of the matrix by adjusting their production, modification or turnover. Accordingly, the importance of specific polysaccharides or modifications is increasingly being associated with specialized functions within seed tissues, often through the spatio-temporal accumulation or remodeling of particular polymers. Here, we review the evolution and accumulation of polysaccharides during eudicot seed development, what is known of their impact on wall architecture and the diverse roles associated with these in different seed tissues.

Seed coats as an alternative molecular factory: thinking outside the box

Seed coats as commodities. Seed coats play important roles in the protection of the embryo from biological attack and physical damage by the environment as well as dispersion strategies. A significant part of the energy devoted by the mother plant to seed production is channeled into the production of the cell layers and metabolites that surround the embryo. Nevertheless, in crop species these are often discarded post-harvest and are a wasted resource that could be processed to yield co-products. The production of novel compounds from existing metabolites is also a possibility. A number of macromolecules are already accumulated in these maternal layers that could be exploited in industrial applications either directly or via green chemistry, notably flavonoids, lignin, lignan, polysaccharides, lipid polyesters and waxes. Here, we summarize our knowledge of the in planta biosynthesis pathways of these macromolecules and their molecular regulation as well as potential applications. We also outline recent work aimed at providing further tools for increasing yields of existing molecules or the development of novel biotech approaches, as well as trial studies aimed at exploiting this underused resource.

The micro- and nanoscale spatial architecture of the seed mucilage-Comparative study of selected plant species

The seed coat mucilage envelope is formed just after hydration and surrounds the seed as a gel-like, transparent capsule. The mucilage envelope represents a special type of modified cell wall with all of the typical polysaccharides i.e. cellulose, pectins and hemicelluloses. The chemical composition of the mucilage is well-recognized but its structural organization remains unclear. In the presented study, we visualized the spatial architecture of the seed mucilage envelope of selected taxa which produce cellulose mucilage. Using critical point drying (CPD) and scanning electron microscopy (SEM) imaging, we demonstrated the structural details of the mucilage from the micro- down to the nanoscale. The mucilage, after CPD, had a visibly spatial structure which differed between the studied taxa; for example, a tangled organization in Arabidopsis thaliana and a more ordered arrangement in Ocimum basilicum were revealed. In general, the mucilaginous fibrillary components formed network made of long, unbranched, thicker cellulose fibrils together with shorter, thinner and, often branched other polysaccharides. Cellulose fibrils built a kind of scaffold for the rest of the components which were spread between them and/or covered their surface. The cellulose fibrils were attached to the seed surface, and therefore prevent the loss of the mucilage envelope during mechanical impacts. The loose architecture and special chemical composition of the mucilaginous cell wall is important for water binding and storage, which are crucial for the proper functioning of the seed mucilage envelope.

Layers of regulation - Insights into the role of transcription factors controlling mucilage production in the Arabidopsis seed coat

A polysaccharide-rich mucilage is released from the seed coat epidermis of numerous plant species and has been intensively studied in the model plant Arabidopsis. This has led to the identification of a large number of genes involved in the synthesis, secretion and modification of cell wall polysaccharides such as pectin, hemicellulose and cellulose being identified. These genes include a small network of transcription factors (TFs) and transcriptional co-regulators, that not only regulate mucilage production, but epidermal cell differentiation and in some cases flavonoid biosynthesis in the internal endothelial layer of the seed coat. Here we focus on the function of these regulators and propose a simplified model where they are assigned to a hierarchical gene network with three regulatory levels (tiers) as a means of assisting in the interpretation of the complexity. We discuss limitations of current methodologies and highlight some of the problems associated with defining the function of TFs, particularly those that perform different functions in adjacent layers of the seed coat. We suggest approaches that should provide a more accurate picture of the function of transcription factors involved with mucilage production and release.